Electronic circuitry has contributed enormously to the advancement of civilization. Designers of electronic circuitry take various constraints into account when conceiving such circuits. One of these constraints is the electromagnetic compatibility (or EMC) of the circuit they are designing. While operating, the circuit should not radiate excessive electromagnetic energy in order not to interfere with the proper operation of other surrounding circuits. Such interference is often termed Electro-Magnetic Interference (EMI).
EMI will radiate to some extent every time a time-dependent voltage or current signal is present on chip. If the EMI is excessive, the functionality of the circuit or its surrounding circuitry may be adversely affected. Also, there are sometimes strict you limits to the amount of EMI that may be emitted as a function of frequency. For instance, in the United States, the Federal Communication Commission (FCC) regulates the usage of certain frequencies of electromagnetic energy.
There are many conventional mechanisms for reducing the amount of EMI emitted by a circuit including the use of passive filters, spread spectrum generation, shielding, and differential signaling.
Passive filtering removes higher frequency harmonics from a signal, thereby degrading signal integrity. Passive components cannot be used to reduce the EMI in the fundamental frequency of a signal unless they reduce the signal amplitude, which is also a trade-off between signal quality and EMI reduction. Spread spectrum generation is an effective solution but there are instances where the deviation in frequency cannot exceed a certain limit and hence spread spectrum generation is typically a trade-off between EMI reduction and functionality of the system. In shielding, the signal is shielded using a metal and dielectric case or layers to contain the electromagnetic waves within a certain physical zone.
Differential signaling and twisted pairs use the concept of electromagnetic field cancelling. However the primary use of the technologies is to preserve signal integrity at higher speeds rather than EMI reduction. The technologies cannot be used to reduce EMI in an existing single-ended system. Furthermore, in differential systems, there is usually still some common node noise that leads to EMI emissions.